The oligomerization of 1-alkenes by acid or Ziegler-Natta catalysis to produce polyalpha-olefin (PAO) synthetic lubricants with superior properties is well known in the art. PAO lubricants are notable in particular for their superior VI and low temperature properties compared to mineral oil based lubes. One characteristic of the molecular structure of 1-alkene oligomers that has been found to correlate very well with improved lubricant properties in commercial synthetic lubricants is the ratio of methyl to methylene groups in the oligomer. The ratio is called the branch ratio and is calculated from infra red data as discussed in "Standard Hydrocarbons of High Molecular Weight", Analytical Chemistry, Vol.25, no. 10, p. 1466 (1953). Viscosity index has been found to increase with lower branch ratio.
Recently, novel lubricant compositions (referred to herein as HVI-PAO) comprising polyalpha-olefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. Pat. applications Ser. No. 210,434 , now U.S. Pat. No. 4,827,073 and 210,435 , now U.S. Pat. No. 4,827,064, both filed June 23, 1988, incorporated herein by reference in their entirety. The HVI-PAO lubricants are made by a process which comprises contacting C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone whereby high viscosity, high VI liquid hydrocarbon lubricant is produced having branch ratios less than 0.19 and pour point below -15.degree. C. The process is distinctive in that little isomerization of the olefinic bond occurs compared to known oligomerization methods to produce polyalpha-olefins using acidic catalyst. Lubricants produced by the process cover the full range of lubricant viscosities and exhibit a remarkably high viscosity index (VI) and low pour point even at high viscosity. The as-synthesized HVI-PAO oligomer has a significant portion of terminal olefinic unsaturation. Typically, the HVI-PAO oligomer is hydrogenated to improve stability for lubricant applications.
Modifications to HVI-PAO oligomers or to prior art PAO synthetic lubes that result in improved thermal stability are particularly sought after as long as those modifications do not result in degradation of other properties such as VI. High VI allows the use of PAO lube stock at high temperature. However, at high temperatures PAO lubricants can break down and lose viscosity. Furthermore, when the lube molecules break down in the presence of oxygen the radical fragments can either combine with each other or react with oxygen to form organic acids and other polar compounds. The result is increased sludge formation and unwanted viscosity increase.
It is an object of the present invention to provide a process for the production of PAO and HVI-PAO lubricants with enhanced thermal stability.
It is another object of the present invention is to provide a process for the production of thermally stable HVI-PAO by structural modification of the HVI-PAO oligomer molecule.
Yet another object of the present invention is to provide a process for the production of thermally and oxidatively stable HVI-PAO by treatment of the HVI-PAO lubricant oligomer with isomerizing agents without significantly degrading the viscometric properties of the lubricant.